The present invention relates to optical detectors, and more particularly to an integrated solid state light-sensing device that discriminates between different polarizations of optical signals, arrays incorporating such light sensing devices and the corresponding methods for using such devices.
Conventional polarized-light detectors will typically require the use of polarizing filters/lenses that are positioned proximate the photodetector to achieve detection of the selected polarization. In this manner, a discrete filter and/or lens is required for each particular polarization detection scheme. As such, these detectors tend to add bulk and size to the overall light detection systems. Additionally, filtered optical signals tend to decrease the degree of sensitivity of the detection process because less of the optical signal ultimately reaches the light-sensing medium in the photodetector.
The need exists to develop a polarized-light detector that will eliminate the need to incorporate external filters and/or lenses, thereby, decreasing the size of such optical detection systems and allowing a greater degree of sensitivity and polarization discrimination capability. Such a device would have widespread application in optical spectrometry, for example, gas sensing or characterization. Additionally, enhancements in data encryption for optical telecommunications could be realized if the polarized-light detector allows for the detection of optical signals transmitted on a selected polarization of light.
Recent advancements in optical communication technology have led to the development of solid state polarized light emitters that incorporate the use of magnetic materials in conjunction with light emitting media. By incorporating these magnetic materials, such as Giant Magnetoresistance (GMR) materials or Dilute Magnetic Semiconductor (DMS) materials, the emitters eliminate the need for optical filters and/or lenses that would typically be used to convert the emitted light to the desired polarization (i.e., right.circularly polarized, left circularly polarized, etc.). In addition, by changing the direction of the applied magnetic field, it is possible to alter the type of polarized light emitted. The resulting emitters occupy less space, are generally more efficient and typically provide for devices that can be manufactured at a lower cost.
A GMR multilayer stack operates under the principle that very large changes in resistance can be realized in materials comprised of alternating very thin layers of various metallic elements. The general structure of GMR multilayer stacks is alternating ferromagnetic and non-ferromagnetic spacer layers, each a few atomic layers thick. The thickness of the spacer layer is such that the magnetic moments of successive ferromagnetic layers are aligned anti-parallel to each other in the absence of an applied magnetic field. It is observed that the resistance of the structure is much higher when the magnetic moments of the adjacent magnetic layers are aligned antiparallel than when they are parallel. Switching from the antiparaliel to the parallel configuration can be achieved by an applied magnetic field. This effect is referred to as giant magnetoresistance (GMR).
Dilute magnetic semiconductors (DMSs), based on manganese doped II-VI and III-V host materials, for example, have recently received a large amount of attention for their unique combination of magnetic and electronic properties. DMSs are formed by substituting a fraction of cations with a magnetic ion. These alloys exhibit a variety of novel magneto-optical properties coming from the exchange interaction between the magnetic ions and the conduction or valence electrons (sxc2x1p exchange interaction).
By incorporating a spin filter layer in a polanrzed-light detector it is possible to eliminate the need to incorporate external filters and/or lenses in the detector construct. This improvement would decrease the size of such optical detection systems and allow a greater degree of sensitivity and polarization discrimination capability.
The present invention provides for an improved integrated solid state light-sensing device that discriminates between different polarizations of light. The device incorporates a spin filter layer between the light-sensing medium and the backside contact of a conventional photodiode structure. Standard polarized-light detectors require the use of polarizing filters and/or lenses that are typically placed in front of the photodetector. The present invention eliminates the need for polarizing lenses and/or filters, thereby decreasing the overall size and complexity of a light detection system. Additionally, by providing for the capability to change the magnetization direction in the spin filter layer, the device""s sensitivity can be altered from a first polarization to a second polarization. The degree of sensitivity of the device should be heightened due to the device""s capability to allow the entire incident optical signal to reach the light-sensing medium unfiltered.
A polarized-light discriminator device according to the present invention comprises a conventional photodiode having a first contact disposed on the backside of a light-sensing medium and a second contact disposed on the frontside of the light-sensing medium. A spin filter medium, typically a Giant Magnetoresistance (GMR) multilayer stack, a Dilute Magnetic Semiconductor (DMS), or a ferromagnetic layer is disposed between the backside of the light-sensing medium and the first contact.
Polarized light incident on the light sensing medium excites electrons with a preferred spin polarization into the conduction band of the light-sensing medium. The spin polarization preference is governed by selection rules for the light sensing medium and the type of light polarization. The application of a reverse bias voltage to the first and second contact causes the optically excited, spin polarized electrons in the light-sensing medium to move toward the spin filter medium and the first contact. In the presence of a magnetic field, typically proximate to the spin filter medium, a net magnetization will be induced in the spin filter medium. The magnetic field may be an external magnetic field, a local magnetic field, or any other magnetic field suitable for inducing a net magnetization in the spin filter medium. The direction of induced magnetization determines the type of spin polarized electrons transmitted (or reflected) by the spin filter medium.
The device typically incorporates an anti-reflective coating layer disposed on the frontside of the light-sensing medium (i.e., the optical signal receiving side of the photodiode) that serves to increase the amount of the incident optical signal that reaches the light-sensing medium. In embodiments in which the second contact is a grid-like array of one or more contact pads the anti-reflective coating layer generally surrounds the one or more contact pads. In embodiments in which the second contact is a transparent conductive layer, such as Indium Tin Oxide or the like, the anti-reflective coating layer may be formed directly on the frontside of the light-sensing medium followed by the transparent conductive layer or the transparent conductive layer may be formed directly on the frontside of the light-sensing medium followed by the anti-reflective coating layer.
In an alternate embodiment, the polarized-light discriminator device of the present invention comprises a light-sensing substrate having semiconductor characteristics, a spin filter layer, typically a GMR multilayer stack, a DMS material or a ferromagnetic material, disposed on the backside of the light-sensing substrate and a contact layer disposed on the spin filter layer. The frontside of the light-sensing substrate has disposed thereon an anti-reflective coating layer and one or more contacts. The application of a magnetic field to the spin filter layer causes the device to discriminate between different polarizations of light. The magnetic field may be external, local or the like, typically proximate to the spin filter layer and is applied to induce a net magnetization in the spin filter layer.
The invention is also embodied in a polarization-selective-light detector array that includes a plurality of photodiodes, each photodiode having a first contact disposed on the backside of a light-sensing medium and a second contact disposed on the frontside of the light-sensing medium. A spin filter medium is incorporated in each of the plurality of photodiodes such that the spin filter medium is disposed between the backside of the light-sensing medium and the first contact. Additionally, a variable wavelength splitter is positioned proximate the plurality of photodiodes for the purpose of segmenting an optical signal into individual wavelengths. In operation, a reverse bias potential difference is applied to the first and second contacts and a magnetic field is applied to the spin filter layer of the plurality of photodiodes to allow the device to determine the polarization states of the individual wavelength or, alternatively, a reverse bias potential difference is applied to select photodiodes within the array to independently address selected elements (i.e., passive matrix addressing). In a typical array, a magnetic field is positioned proximate to the array to induce a net magnetization in the spin filter medium of the plurality of photodiodes, thus allowing only spin compatible electrons to be transmitted to the first contact. Additionally, an anti-reflective coating layer will typically be disposed on the frontside of the light-sensing medium of each photodiode construct. The anti-reflective layer may be formed to surround the second contact of the photodiode in those embodiments in which the second contact comprises a grid-like array of contact. Conversely, in those embodiments in which the second contact comprises a layer of transparent conductive material, the anti-reflective layer will typically be formed directly on either the light-sensing medium or the transparent conductive material.
Further, the invention is embodied in a method for using the polarized-light discriminator device of the present invention. The method comprises the steps of transmitting a polarized optical signal into a polarized-light detector having a spin filter medium disposed on the backside of a light-sensing medium, a first contact disposed on the spin filter medium and a second contact disposed on the frontside of the light-sensing medium. The polarized optical signal excites spin-polarized electrons into the conduction band of the light-sensing medium. The application of a reverse bias current to the first and second contacts moves the spin-polarized electrons toward the spin filter medium and the first contact. A magnetic field is applied to induce net magnetization in the spin filter medium. The electrical field in conjunction with the magnetic field causes transmission of select electrons through the spin filter medium based upon the spin orientation of the electrons. In one embodiment of the invention the output current of the transmitted selected electrons is detected at the first contact.
In accordance with yet another embodiment of the invention, a method for selective optical wavelength detection comprises splitting a multiple wavelength optical signal into individual wavelength segments and transmitting the individual wavelength segments into an array of photodiodes. Each photodiode having a spin filter medium disposed on the backside of a light-sensing medium, a first contact disposed on the spin filter medium and a second contact disposed on the frontside of the light-sensing medium. Spin-polarized electrons are then excited within the light-sensing medium into the conduction band of the medium by the transmitted polarized optical signal. The application of a reverse bias current to the first and second contacts moves the excited spin-polarized electrons toward the spin filter medium and the first contact. A magnetic field is applied to induce net magnetization in the spin filter medium. The electrical field in conjunction with the magnetic field causes transmission of select electrons through the spin filter medium based upon the spin orientation of the electrons. The select electrons are then detected at the first contact by measuring the current output.
The light-sensing device of the present invention provides for the capability to discriminate between different polarizations of optical signals. In doing so it eliminates the need to provide for extraneous polarizing filters and/or lenses, thereby decreasing the size of known devices. Additionally, the present invention provides for a light polarization sensitive device that can be switched from one polarization to another by switching the electron spin that is transmitted through the spin filter layer.